1. Field of the Invention
This invention relates to lighting units, which are well suited for use with solid state Lighting sources, such as light emitting diodes (LEDs), and more particularly to LED based low profile lighting units for different illumination applications such as illuminating channel letters.
2. Description of the Related Art
Recent developments in LEDs have resulted in devices that are brighter, more efficient and more reliable. LEDs are rugged, consume less power, have a relatively long life (up to 100,000 hours), operate at low voltage, and are 30 to 70% more energy efficient than conventional lights, such as incandescent, neon or fluorescent bulbs. As a result of these developments, LEDs are becoming utilized in many more lighting applications that were previously the realm of incandescent, neon or fluorescent light sources.
Channel letters are commonly found on the outside of buildings and are often used to advertise the name of the business. They are typically constructed of aluminum or plastic housing having the shape of a letter and are approximately 5″ deep, while low profile channel letter housings are less than 5″ deep. The housing has a generally U-shaped cross-section, with the top opening in the housing covered by a colored translucent lens that transmits light from within the housing. Low profile channel letters are being used in different settings, such as indoors or where physical space is limited.
Channel letters are typically illuminated with neon or fluorescent light sources that are mounted within the channel letter housing. Neon and fluorescent lights provide a bright and continuous light source that allows the channel letters to be visible at night. These light sources, however, have a relatively short life (20,000 hours), are fragile, operate at high voltage (7,000 to 15,000 volts for neon) and can consume a relatively large amount of power. Neon bulbs can also experience difficulty with cold starting, which can lead to the bulb's failure. These light sources can be bulky such that they would not be able to be used in low profile channel letter housings. Additionally, neon and fluorescent lights have rigid housings that are shaped to fit the desired application. Neon and fluorescent lights do not have the flexibility of being able to be manipulated and/or adjusted to accommodate for different applications after the shape of the housing has been formed.
Conventional channel letter lighting units are typically made of a rigid PCB within a housing, while other conventional channel letter lighting units are arranged as a long, flexible ribbon. Conventional rigid modules work well in shallow channel letters as long as the overall face of the letter is quite large. Shallow channel letters that are either very small overall or have a narrow cavity that allow only a single row of modules to be installed can appear blotchy and uneven when illuminated with conventional channel letter rigid modules. This trend holds for nearly every type of channel letter including: face-lit, back-lit or halo, and solid-lens acrylic letters. Because they are rigid, these modules are not ideal to install on tightly curved walls of small channel letters.
Conventional flexible ribbon-style products solve many of the above problems, encountered by rigid modules, as they typically feature more lower-powered LEDs spaced closely together. The closely spaced LEDs solve the blotchy appearance of many applications. However, the flexible ribbon products are limited in two key ways. The first is that they only flex in one plane. This does not limit their use on the walls around the perimeter of channel letters. However, if the channel letter shape or application is more conducive to mounting the LEDs on either the face or bottom of the letter, the flexible ribbon must be cut and spliced into several short, straight sections. This is analogous to attempting to create a circular pattern of tape on a flat piece of paper; an impossible feat without cutting the tape into short sections. The need for an installer to cut and splice sections of ribbon product together makes apparent the second key limitation of flexible ribbon products. To connect the ribbon the installer must cut and strip two pieces of wire to solder between each connection. This process is quite tedious and requires soldering implements. The over molded versions of LED ribbon products require the additional step of scraping the overmold off to expose the solder connection point. Both types of ribbons leave the solder joint exposed unless the installer takes yet another additional step of coating or covering the joint.
For example, FIG. 1 shows a small, back-lit channel letter unit 10 with a very narrow cavity 12. The channel letter 10 only has enough room for 5-10 of the smallest conventional channel letter modules. Those modules do not spread emitted light sufficiently to provide an even light distribution. The ideal way to propagate the channel letter unit 10 with conventional channel letter products is with an LED ribbon device on the bottom flat surface 14. In order to ensure proper installation, the installer must cut and splice 2 or 3 sections together.
FIG. 2 shows another conventional shallow back lit channel letter unit 20 with a narrow cavity 22 propagated with small conventional channel letter modules 24. The configuration shown in FIG. 2 does not provide even light distribution, because the channel letter modules 24 are too far apart to blend evenly in such a shallow channel letter unit 20. The modules 24 occupy too much space to allow the light to properly reflect off the inner surfaces 26 of the channel letter 20. A better way to propagate the channel letter unit 20 is to use an LED ribbon product 30 mounted to either the wall 27 or bottom 29 of the channel letter unit 20, as shown in FIGS. 3 and 4. Mounting a ribbon product 30 to the wall 27 is relatively simple, as it only requires one continuous strip. FIG. 3 shows an example of the conventional LED ribbon product 30 mounted to the wall 27 of the channel letter unit 20.
FIG. 4 shows an example of the conventional LED ribbon product 30 mounted to the bottom 29 of the channel letter unit 20. A disadvantage of mounting the LED ribbon product 30 to the bottom 29 of the channel letter unit 20 is that the LED ribbon product 30 must be cut and spliced forming multiple sections of the ribbon product 30, in order to provide illumination to the channel letter unit 20. This increases the complexity of the installation as well as the time and expense of installing the LED ribbon product 30.
LEDs have certain characteristics that make them desirable for many lighting applications that were previously the realm of incandescent or fluorescent lights. Incandescent lights are very energy-inefficient light sources with a vast majority of the electricity they consume being released as heat rather than light. Fluorescent light bulbs are more energy efficient than incandescent light bulbs, but are still relatively inefficient. LEDs by contrast, can emit the same luminous flux as incandescent and fluorescent lights using a fraction of the energy.
In addition, LEDs can have a significantly longer operational lifetime. Incandescent light bulbs have relatively short lifetimes, with some having a lifetime in the range of about 750-1,000 hours. Fluorescent bulbs can also have lifetimes longer than incandescent bulbs such as in the range of approximately 10,000-20,000 hours, but provide less desirable color reproduction. In comparison, LEDs can have lifetimes between 50,000 and 70,000 hours. The increased efficiency and extended lifetime of LEDs is attractive to many lighting suppliers and has resulted in LED lights being used in place of conventional lighting in many different applications. It is predicted that further improvements will result in their general acceptance in more and more lighting applications. An increase in the adoption of LEDs in place of incandescent or fluorescent lighting would result in increased lighting efficiency and significant energy saving.
The increased efficiency and extended lifetime of LEDs is attractive to many lighting suppliers and has resulted in LED lights being used in place of conventional lighting in different sign applications. For example, U.S. Pat. No. 5,697,175 to Schwartz, discloses a low power illuminated sign that is particularly adapted for use with common EXIT signs over doorways. The back of each sign comprises a reflector with a series of cavities with curved surfaces. Each cavity corresponds to a letter and background area in the sign. LEDs are mounted in the center of the cavities to illuminate the letters or background area. The LEDs are provided on a separate perpendicular circuit board or on a central projection formed in the bottom of the cavities, with light from the LEDS directed outward. The letters and background area of the sign are illuminated by light reflecting forward from the curved surfaces of the cavities, so that the only visible light is from the illumination of the cavities.
U.S. Pat. No. 6,042,248, to Hannah et al., discloses an LED assembly for channel letter illuminating signs having an enclosure/housing covered by a translucent lens. Each sign includes a plurality of track moldings at the base of its enclosure, with the moldings running along the longitudinal axis of the sections of the channel letter. Linear arrays of LEDs are mounted on printed circuit boards (PCBs) that are then mounted in the track moldings. Each track molding can hold two PCBs in parallel with each of the PCBs arranged on a longitudinal edge, with the LEDs directed outward.
U.S. Pat. No. 4,439,818 to Scheib discloses a lighting strip that utilizes LEDs as the light source. The strip is flexible in three dimensions and is useful in forming characters and is capable of providing uniform illumination regardless of the characters selected for display. The strip comprises a flexible multi-layered pressure sensitive adhesive tape, having a plurality of triangle cutout sections on each side of the tape, with LEDs connected in a series with a resister. One disadvantage is that this arrangement is not durable enough to withstand the conditions for outdoor use. The flexible tape and its adhesive can easily deteriorate when continually exposed to the elements. Furthermore, this strip cannot be cut to different lengths for different, custom applications.
U.S. Pat. Nos. 6,932,495 and 7,241,031, both to Sloan et al., disclose channel letter lighting units and lighting systems utilizing the lighting units. In some embodiments these lighting units can be provided as multiple lighting units interconnected by conductors in a chain so that an electrical signal applied to the chain causes the lighting units to emit light. The chain can be made available to purchasers on different holding devices such as a box, reel or rack. Different lengths of the chain can be utilized for a particular channel letter, with the desired length of chain being cut from the holding device and mounted within the channel letter. Power can then be applied to the chain in the channel letter causing the units to emit light.
Different types of chains can have different numbers of lighting units per a length, or stated differently, a different density of lighting units. These chains are typically sold at a cost per measure of length, and the cost per length is typically greater for lighting systems having higher density. To accommodate the different needs of customers for chains of different densities, many different types of lighting system chains need to be maintained and stored and made available to customers. In some channel letter applications it may be desirable to have different densities of units in different locations. This can require purchasing multiple chains with different densities for the same job.
It is desirable to provide a lighting unit that is flexible, has a relatively low profile, and can be customized to fit and be mounted on a variety of different structures. As part of this ability to customize, it is desirable to provide a lighting unit that can be cut on location without compromising the function of the underlying light emitting devices.